In many instances, it is desirable to control the relative movement between two articles by permitting or preventing the movement of one article relative to the other. One such instance is described in U.S. Pat. No. 4,949,650 to Allard which is incorporated herein by reference thereto. In this patent, one part of a table top is moved relative to a fixed support and the other part of the table top for various purposes. For some purposes, it is desirable to have apparatus to permit a slidable member to be moved in one direction by simply applying a force thereto but to prevent movement automatically in the opposite direction. For other purposes, it is desirable to prevent movement of the slidable member until desired. In all instances, it is desirable that the movement or non-movement be accomplished as easily and economically as possible.